docs: def search_ss_bonds(self, threshold=3.0):
          """ Searches S-S bonds based on distances
              between atoms in the structure (first model only).
              Average distance is 2.05A. Threshold is 3A default.
              Returns iterator with tuples of residues.
          """
          
          # Taken from http://docs.python.org/library/itertools.html
          # Python 2.4 does not include itertools.combinations
  
          def combinations(iterable, r):
              # combinations('ABCD', 2) --> AB AC AD BC BD CD
              # combinations(range(4), 3) --> 012 013 023 123
              pool = tuple(iterable)
              n = len(pool)
              if r > n:
                  return
              indices = range(r)
              yield tuple(pool[i] for i in indices)
              while True:
                  for i in reversed(range(r)):
                      if indices[i] != i + n - r:
                          break
                  else:
                      return
                  indices[i] += 1
                  for j in range(i+1, r):
                      indices[j] = indices[j-1] + 1
                  yield tuple(pool[i] for i in indices)
  
          
          
          model = self.child_list[0]
          cysteines = [r for r in model.get_residues() if r.get_resname() == 'CYS']
  
          pairs = combinations(cysteines, 2) # Iterator with pairs
  
          for cys_pair in pairs:
              if cys_pair[0]['SG'] - cys_pair[1]['SG'] < threshold:
                  yield cys_pair